Triggering an action based on the "User Status Set Date"

Hi Dear Friends,

We have the following requirement in one of our scenario.

Customer is going to the online web shop and creating an "Sales Order" which will be in "Open" Status. 

Now after "X" number of days he may access the same order which he has created and change the status as "Final Order" then an action should be triggered and a mail should be sent to customer on user status set date + 21 days.

Please let me know how to do it....

Thanks & Regards,

Raj